The present invention relates to connecting a data unit, for example a modem, to a switched telephone network and in particular to an apparatus which facilitates the selection of one from a plurality of modems for connection to said network.
Systems in which one of a plurality of modems may be connected to a telephone line are known, for example as shown in Bell Laboratories Record, Volume 54, No 11 of 11 December 1976. A system is disclosed in which the normal mode of operation is via private circuits and the telephone line provides a back-up facility. The modems are connected via a central switching matrix and a suitably sized matrix must be provided for each specific configuration.
The use of a common dialing device is shown in U.S. Pat. No. 4,125,872 of Maxwell. However in this arrangement each modem has a dedicated telephone line therefore the problem of switching the line between modems does not arise.